The present invention relates to the field of the processing of laser light and more particularly to frequency doubling of such light.
Existing laser sources do not provide adequate coverage of the optical spectrum. Consequently, it is typically useful to double the frequency of a laser, to sum the frequency of two different lasers, to produce new frequency sources, or to parametrically generate new frequency sources. The efficiency of these processes is typically less than 50% in most commercial systems available today. Thus, it is desirable to provide a technique which will improve the frequency conversion efficiency of second harmonic generators (SHG), sum frequency generators (SFG), optical parametric oscillators (OPO), and optical parametric amplifiers (OPA). In accordance with the present invention, this technique comes about by increasing the nonlinear interaction length of an SHG, SFG, or OPO process. This technique can be applied to systems which are critically, noncritically, or quasi-phase-matched and could also be used for a nonlinear frequency conversion process such as frequency up conversion where one of the sources is a laser and the other source is incoherent.
Increasing either the intensity of the laser source or increasing the nonlinear medium length can be used to achieve increased nonlinear conversion efficiency. Current methods for achieving these conditions include increasing the intensity. The intensity can be increased by using a more powerful pump laser source or by focusing the beam more tightly into the nonlinear medium. There are however, practical limits to how much power a given type of laser source can achieve. Focusing tightly has limited usefulness since diffraction effects cause the length of the focal region to decrease at the same rate that the intensity increases. Also, for some systems, the damage threshold intensity for the nonlinear medium is less than the intensity required for high efficiency nonlinear interactions.
Another technique for increasing conversion efficiency is to increase the interaction length. In most high power frequency conversion applications the nonlinear medium is a birefringent crystal that is cut at the proper angle such that the pump and generated frequency wavefronts maintain the phase-matching condition as they copropagate through the crystal. However, the length of these crystals is limited by the state of the art of crystal manufacturing processes. In most cases this is less than a few centimeters. Furthermore, for critical phase matching in birefringent crystals, different frequency beams propagate through the crystal at different directions, a phenomenon referred to as walk-off. Walk-off limits the effective interaction length to roughly the beam diameter divided by the walk-off angle.
Thus, in view of the foregoing, the nonlinearity cannot be arbitrarily increased due to: limited nonlinear medium length; limited laser power; and walk-off effects. It is for this reason that we have invented a technique to increase the interaction length of nonlinear interactions such as SHG and SFG by coupling two or more crystals together in the same system. Previously, the efficiency achieved using single-pass nonlinear frequency conversion has been limited by crystal length, intensity, and walk-off issues. For lasers whose peak output power is adequate to achieve high intensities without focusing (such as Q- switched lasers), high efficiencies of 60% or more can be achieved. However when focusing is required, such as in the cases of cw or cw-mode-locked lasers, the doubling and sum-frequency conversion efficiencies are typically no more than 25%. In the case of cw lasers, the limiting factors are short focal depth or inadequate laser intensity.
Alternate methods of enhancing SHG efficiency include constructing an optical cavity around the nonlinear medium. The cavity circulates the non-converted pump power so that the pump power inside the cavity builds up to a higher value than the incident pump power and the pump photons pass more than once through the nonlinear medium. The intracavity power will increase until the total cavity loss limit is reached. If the linear cavity loss can be much less than the single pass conversion, then high efficiency doubling can be achieved in this manner. However, this method involves complicated engineering to match the external cavity to the laser cavity. Optical cavity lengths must be matched to within a fraction of the wavelength of the pump radiation. Typically, a closed-loop servo system is required to achieve this cavity length-matching condition. Such systems are often complicated and susceptible to mechanical instabilities. To avoid constructing a second cavity, the nonlinear medium may be placed inside the pump laser cavity. The output coupling mirror of the cavity is replaced with a dichroic mirror which will out-couple all of the second harmonic and totally reflect the fundamental pump radiation. In theory, the nonlinear medium length and focusing parameters can be adjusted to convert all the pump radiation which would have exited the original cavity into the second harmonic. However, loss effects can prevent this from happening, and temporal instabilities typically result from intracavity doubling. Furthermore, the efficiency for intracavity doubling can only be optimized for a particular output power level since the SHG process with the dichroic mirror is analogous to the output coupling in a simple laser cavity. In most practical cases, a pump laser source of two to four times the intensity of the desired frequency source must be developed in order to achieve adequate power at the desired frequency. Development of an oversized pump laser typically represents increased expense, power consumption, and cooling requirements.